Quadrature multi-frequency ranging (QMFR) applied to GPS multipath mitigation

ABSTRACT

A quadrature multi-frequency ranging system applied to DSSS multipath mitigation problems is described. The QMFR techniques include taking the time-of-arrival measurements on the received signal at a time when the multipath component is at a 90 degree angle to the direct path. Further, the preferred embodiment of the present invention recognizes that when a signal is transmitted at a series of frequencies, and the spread of these frequencies is sufficient, a quadrature or near quadrature condition can be forced to occur, at which the TOA estimation gives the highest certainty of measurement.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/190,606, filed Jul. 9, 2002, which is a continuation-in-part ofpatent application Ser. No. 09/365,702, filed Aug. 2, 1999, and whichclaims the benefit of provisional application Ser. No. 60/303,511 filedon Jul. 9, 2001.

FIELD OF THE INVENTION

The present invention relates to satellite communications and moreparticularly to Global Positioning System (GPS) or other Direct SequenceSpread Spectrum (DSSS) multipath mitigation techniques.

BACKGROUND AND SUMMARY OF THE INVENTION

DSSS systems include GPS transmitter/receiver systems that performtime-of-arrival (T.O.A.) determinations between a DSSS transmitter (suchas a GPS satellite) and a DSSS receiver (such as GPS user equipment).The present invention applies to all DSSS Systems, but will be describedfor convenience with request to a GPS system herein. Multipath problemswithin GPS systems are well known and take a variety of different formsincluding ground reflection, single reflection (diffraction), diffusemultipath, and mobility-induced errors. Each of these is discussedbelow:

(1) Ground reflection. A strong multipath signal between a GPS satelliteand a GPS user equipment (for example 100% reflected power) canoriginate from ground or sea surface reflections, as shown in FIG. 1. Ifthe angle E is a low elevation angle and/or the user altitude h is low,the multipath may be only slightly delayed from the main path betweenthe GPS satellite and the user equipment. If the multipath delay is lessthan 1.5 chips, the multipath delay degrades the signal TOA estimation.For C/A-code, this condition occurs for:Delay =ΔR/c=2h sin E/c<1.5 μs.

Ground or sea surface reflections and the effects thereof are describedin Spilker, “Overview of GPS Operation and Design,” American Instituteof Aeronautics and Astronautics, Inc. Vol. 163 at page 53 (1994). Atypical behavior of the ground reflection error after smoothing as thesatellite travels across the sky is described in Brenner, “GPS LandingSystem Multipath Evaluation Techniques and Results,” at pages 1001-2.

(2) Single Reflector (or Diffractor). Another multipath problem occurswhen a single reflector or diffractor creates a multipath signal betweenthe user equipment and the GPS satellite. Such a multipath problem isillustrated in FIG. 2, where the main path signal (on top of FIG. 2) andthe multiple path (on the bottom of FIG. 2) between a user equipment andGPS satellite are shown being created by the diffractor. Brenner, “GPSLanding System Multipath Evaluation Techniques and Results,” ION GPS-98,at pages 1000-1001 (Conference on September 15-18, 1998) also describesthe resultant multipath-induced position error, which is reproduced byway of example in FIG. 3.

(3) Diffuse multipath. Another type of multipath problem occurs withuniformly scattered diffractors, as shown in FIG. 4. In the case ofuniformly scattered diffractors, the power equals P_(diffr) (0.2/dr²).The smoothed multipath error in the case of diffuse multipaths isdescribed in Brenner at pages 1000-1001.

(4) Effect of mobility. Still another multipath problem that existsbetween GPS satellites and user equipment occurs as a result of mobilityin the user equipment. According to Van Nee, “Multipath Effects on GPSCode Phase Measurements,” Navigation, Vol. 39, No. 2, Summer 1992 atpages 179-180, the motion of the user equipment causes differences inCarrier Doppler frequencies between reflections in the line of sight ofstationary users versus mobile users. That is, the fading bandwidth isdetermined by the Doppler differences, which varies substantially forstationary users versus mobile users. Further, reduction of themultipath error variance requires an averaging time much greater than1/fading bandwidth. For stationary users, maximum Doppler difference is0.6 Hz (and most users even experience much lower values). Mobile users,however, experience much higher fading bandwidth, e.g., for v=15 m/s,Doppler differences can take values up to 180 Hz. As a result, smoothingtechniques require long time constants (on the order of 100 seconds) forstationary users but can use much shorter time constants for mobileusers.

Still further multipath signal generation can be caused by terrain (suchas) urban canyons and by signal reception within buildings.

There are current receiver technologies that attempt to mitigate themultipath disorders. Current GPS transmitter/receiver systems attempt toperform time of arrival determinations on the GPS spread spectrumwaveform by (1) correlating the received signal with a replica of thetransmitted signal and then (2) finding the time location of the peakmagnitude of the correlation. They either locate the peak directly or bycurve-fitting an ideal correlation function (a triangular pulse) withthe actual received signal correlation function. Three recent systemsfor receiver technology improve the TOA estimation accuracy of the GPSreceiver in a multipath environment:

(a) Narrow correlator (Novatel™). The narrow correlator uses acorrelator spacing of a fraction of a chip rather than chip-spacedcorrelators, as illustrated in FIG. 5. Using a fraction of a chipgreatly reduces the magnitude of the maximum TOA error in a multipathenvironment. The error reduction by the narrow correlator is describedby Van Dierendonck et al., “Theory and Performance of Narrow CorrelatorSpacing in a GPS Receiver,” Navigation, Vol. 39, No. 3, Fall 1992, atpages 265-283 and reproduced by way of summary in FIG. 6. In FIG. 6, themultipath error for a single diffractor, with narrow correlation isillustrated.

Others have extended narrow correlators to P(Y) code receivers, e.g.,Karels, et al. “Extending Narrow-Correlator Technology to P(Y)-CodeReceivers: Benefits and Issues,” ION GPS-94 Sep. 20-23, 1994,investigated using narrow-correlator techniques on P(Y)-code receivers.Karels et al opined that improvements in overall GPS receiverperformance obtained in commercial GPS receivers over standard C/A-codereceivers may be extended to P(Y)-code GPS receivers but only if the GPSspace vehicle (SV) spectral output is permitted to increase. A table ofKarels, et al., finding is reproduced as FIG. 8.

(b) Multipath estimating delay lock loop (MEDLL) (Novatel™). MEDLL makesmultipath error correction by assuming that no more than two dominantmultipath signals are present. It estimates the amplitude, delay, andphase of each multipath component using maximum likelihood criteria andthen subtracts each estimated multipath correlated function from themeasured correlation function. The remaining direct path correlationfunction has minimal multipath degradation, and it can be used foraccurate TOA estimation. The technique is described by Towsend, et al.,“Performance Evaluation of the Multipath Estimating Delay Lock Loop,”ION National Technical Meeting Jan. 18-20, 1995, and exhibits multipatherror correction illustrated by way of summary in FIG. 7. Inpracticality, the MEDLL technique gives performance that's comparable toa p-code receiver.

(c) Leading edge curve fitting. A third system for improving TOAestimation accuracy of a GPS receiver in a multipath environment is theleading edge curve fitting technique first used by the present assigneefor W-sensor applications and for small unit operations programs. Theleading edge curve fitting technique matches the receive signalcorrelation with an ideal correlation function on the leading edge ofthe received signal correlation. This minimizes the impact of anydelayed multipath signals when computing the TOA, because the multipathhas its greatest influence on the trailing edge of the correlation.

The preferred embodiment of the present invention provides a more usefulreceiver technology for improving the TOA estimation accuracy byutilizing a QMFR technique in the GPS application. This techniquereduces the influence of close-in multipath components by examining thecomplex correlation function of the received signal rather than just themagnitude of the correlation function. It uses curve fitting on thecomplex correlation signal to locate the correlation peaks due to themain (undelayed) path in the delayed multipath component, and thenmeasures the phase angle between those peaks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art illustration of the multipath problem addressed bythe present invention;

FIGS. 2-3 are prior art illustrations of the single reflector multipathproblem;

FIG. 4 illustrates the diffuse multipath problem;

FIGS. 5-6 address the prior art narrow correlator technique to mitigatemultipath problems;

FIG. 7 illustrates multipath error correction by the prior art multipathestimating delay lock loop receivers;

FIG. 8 illustrates the effect of narrow correlator techniques onP(Y)-code receivers;

FIGS. 9-19 illustrate a presently preferred technique for recognizingquadrature condition in most GPS receivers in accordance with an exampleaspect of the present invention;

FIG. 20 illustrates a receiver embodiment in accordance with an exampleembodiment of the present invention;

FIG. 21 is a table of example benefits of embodiments of the presentQMFR system;

FIG. 22 is a table of recommended approaches for multipath mitigation inaddition to narrow correlation; and

FIG. 23 illustrates QMFR experimental effectiveness in addressing themultipath problem.

DETAILED DESCRIPTION OF A PRESENTLY PREFERRED EMBODIMENT

As shown in FIG. 9, the TOA estimate is degraded by interference ofdelayed multipath signals with the correlation peak due to the undelayedmain path signal, shown in FIG. 9. This is particularly true formultipath signals that are delayed by less than 1.5 times the chipduration. Receivers other than the present invention, such as the narrowcorrelator, MEDLL, and leading edge curve fitting embodiments, do notimprove performance as well as the present invention. Even when theabove mentioned curve fitting on the leading edge of the received signalis used as a correlation function, the influence of a close-in multipathcomponent is still severely degrading as to the TOA estimates. Thenarrow correlator and MEDLL techniques, although they significantlyimprove performance against multipath, still do not take advantage ofthe quadrature condition, as described below. Moreover, to extend thenarrow correlator technique to work for P(Y)-code and M-code wouldrequire a large bandwidth increase in order to achieve significantlyimproved multipath performance.

Thus, the presently preferred embodiment of the present inventionutilizes the QMFR technique and extends it to application of the GPSmultipath problem. The QMFR technique reduces the influence of close-inmultipath components by examining the complex correlation function ofthe received signal rather than just the magnitude of the correlationfunction. Thus, as shown in FIG. 10, the QMFR concept utilizes thecomplex correlation function in which the main path and multipathcomponents arrive with arbitrary phase angles in the complexcorrelation. The preferred embodiment uses curve fitting on the complexcorrelation signal to locate the correlation peaks due to the main(undelayed) path and the delayed multipath component, and then measuresthe phase angle between these peaks.

The preferred embodiment of the present invention applies first andsecond QMFR principles. The first QMFR principle (Q1) supposes that thebest TOA estimate on the main path can be obtained when the delayedmultipath component is at a 90 degree angle to the main path, as shownin FIG. 19. Recognizing this first principle, the present inventionutilizes the understanding that the main path and multipath componentsarrive with arbitrary phase angles in the complex correlation and thatcertain phase angles have less interference by the multipath componenton the main path's leading edge. The present inventors have found thatthe best angle is 90 degrees and thus the QMFR concept of the preferredembodiment waits for this 90 degree angle condition to occur and thenmeasures the TOA.

Thus, the preferred embodiment detects an instance when the 90 degree or“quadrature” condition occurs, and then does leading edge curve-fittingon the main path in the complex domain. This results in significantlyhigher TOA accuracy than leading edge curve fitting on the magnitude ofthe correlation function. Locating and utilizing the quadraturecondition also gives improved performance over narrow correlator andMEDLL, which operate at arbitrary phase angles between the direct andmultipaths. Also there is no increase in bandwidth necessary to use thequadrature condition for P(Y)-code or M-code, unlike what is requiredwith narrow correlation.

FIGS. 11-18 are graphical representations of the advantages gained bythe present invention. FIG. 11 and 12 are, respectively,three-dimensional and two-dimensional displays of the correlationfunction with the multipath delayed by 1.2 chips (using C/A code). Inthis embodiment, the angle between the main path and the multipath is 0degrees. As shown, the constructive interference between the multipathand main path causes an early leading edge curve fit TOA estimate.

FIGS. 13 and 14 are similar three-dimensional and two-dimensional graphsillustrating the correlation function when the angle between the mainpath and the multipath is 90 degrees. As shown particularly in FIG. 14,the maximum amplitude of the main path occurs with no interference fromthe multipath at the angle of 90 degrees.

By comparison, FIGS. 15 and 16 are, respectively, three-dimensional andtwo-dimensional graphs where the angle between the main path andmultipath is set at 120 degrees, rather than the 90 degrees shown inFIGS. 13 and 14. At 120 degrees, the main path amplitude is reduced, asparticularly shown in FIG. 16.

Finally, FIGS. 17 and 18 are three-dimensional and two-dimensionalgraphs, respectively, where the angle between the main path andmultipath is 180 degrees. As shown in FIG. 18, destructive interferenceoccurs between the multipath and main path causing a late leading edgecurve fit TOA estimate.

FIGS. 11 through 18 illustrate the optimal advantages gained when themultipath component is at 90 degrees to the main path. The phase anglebetween the main or multipath (delayed) signal is of course arbitrary,so only on occasion will this quadrature condition occur. The preferredembodiment of the present invention waits for the quadrature conditionto arbitrarily occur and then does leading edge curve fitting on themain path in the complex domain based on the signals obtained at thegeneral occurrence of the quadrature condition. As shown in FIG. 19, the90 degree angle between the main path and multipath gives the lowest TOAstandard deviation. In theory, the quadrature condition can provide aTOA estimation brought to within a few dB of the Cramer-Rao bound. Thepresent invention, as shown in FIG. 19, can avoid TOA estimationdegradations of 5-10 dB due to worst case interference between the mainpath and the multipath. In the example of FIG. 19, the chip duration wasset at 98 microseconds, C/NO=55 dB, and the number of trials was 100. Asshown, the worst case interference occurred near zero degrees and 180degrees, and the lowest TOA standard deviation occurred when the mainpath and the multipath were 90 degrees to each other.

The preferred embodiment of the present invention in which thequadrature condition is awaited provides improved accuracy over thenarrow correlator or MEDLL prior techniques, which operate at allarbitrary phase angles between the direct path and the multipath.Further, the presently preferred embodiment can employ even more rapidmeasurements than required for smoothing across phase angles for severalcycles. Further, extension of improved accuracy of P(Y)-code or M-codereceivers is also possible with the present invention, without demandinglarge increases in SV transmit bandwidth.

In an alternative embodiment, the present invention relies upon a secondQMFR principle (Q2) as well, namely if a signal is transmitted at aseries of frequencies, and the spread of these frequencies issufficient, a quadrature (or near-quadrature) condition can be forced tooccur. Recognizing from the first principle, that the quadraturecondition is the optimum condition for obtaining the leading edge curvefitting on the main path in the complex domain, the principle of Q2seeks to force that advantageous condition to occur. This principlerecognizes that the angle between the main path and the multiple pathwill differ at different frequencies, but if the signal is transmittedat a series of frequencies and the spread of those frequencies issufficient, the quadrature condition (or near-quadrature condition) willbe forced to occur. For this, the required frequency spread is:where Δf is the required frequency spread and At is the delay betweenthe main path ${\Delta\quad f} = \frac{1}{4\Delta\quad t}$and the multipath. For example, to achieve quadrature condition for amultipath delay as small as ¼ chip of M-code would suggest:Δt=(¼)(1/(5 MHz))=50 ns, andΔf=1/(4*50 ns)=5 MHz.

This principle is applied by having the transmitter frequency hop at aseries of frequencies that span the required frequency spread. Then, asuitably-equipped receiver receives the GPS signal on all thesefrequencies, looks at the phase angle between the direct and multipaths,and chooses the hop frequency which comes closest to achieving thequadrature condition. A TOA measurement made on this frequency will havethe best separation between the direct path and the multipath, and onaverage will have the lowest TOA error.

Thus, in the waveform transmitter, which may be either a space vehicle,or pseudolite, the QMFR implementation is as follows:

(1) for C/A-code: the waveform frequency hops over F_(c)+/−1 MHz for ¼chip MP.

(2) for P(Y)-code: the waveform frequency hops over F_(c)+/−5 MHz for ¼chip MP.

(3) for M-code: the waveform frequency hops over F_(c)+/−2.5 MHz for ¼chip MP.

For P(Y) and M-code QMFR in space vehicles, the resultant waveform codeexceeds the spectral allocation for GPS. Spectral spillover for M-codewith QMFR will be limited to a small region because of the spot beam.The spot beam is a special focused energy beam transmitted from aspecial antenna located on the satellite, as opposed to the standardearth coverage GPS signal.

In pseudolite transmitters, pulsed operation minimizes interference withthe signal from the space vehicle and reduces the effect of anyspillover into frequency allocations other than those allocated toglobal navigation satellite system (GNSS).

FIG. 20 illustrates an example embodiment of a receiver employingaspects of the present invention. The receiver receives base band GPSsignals “RXI” and “RXQ” and inputs them respectively into the chipmatched filters (CMFs), which input them to the Digital Matched Filters(DMF), for correlation with a replica of the digital transmissionsignal. The output of the DMF is the complex correlation function DMFIand DMFQ. The complex correlation function is provided to the curvefitting algorithm portion, which also receives the theoreticallyreceived pulse shape. The curve fitting algorithm outputs main and multipath signals to the “select quadrature condition” portion (TOA andangle) where the quadrature condition is detected and timed forobtaining the TOA estimate. The select quadrature condition portionapplies the principles of QMFR to obtain the main path TOA, inaccordance with the principles previously identified.

The transmitter components include a standard GPS signal generator, RFfrequency hopping device, which offsets the frequency of the transmittedGPS signal, and (for pseudolites only) a switching device to turn thetransmitter rapidly on and off for pulsed operation.

The receiver components, shown in FIG. 20, include the RF frequencyhopping device, which offsets the frequency of the received GPS signalin synchronization with the frequency hopped transmit signal. Also, forreception of pseudolites only, a switching device to turn the receiverrapidly on and off in synchronization with the pulse transmitter isrequired. Then, the correlator device is included in the receiver thatgenerates the base band in-phase and quadrature components of thecorrelation between the received signal and a replica of the digitaltransmitted signal (in DMF, FIG. 20). The curve fitting device thenimplements the algorithm to determine amplitude, delay, and phase angleof (i) the direct signal and (ii) the largest of the delayed (multipath)signals. This occurs in the curve fitting algorithm box of FIG. 20.Finally, the receiver includes the selection device which decides whichfrequency hop comes closest to yielding the quadrature condition, and aleading edge curve fitting algorithm that performs the TOA estimation.This is included in FIG. 20 in the select quadrature condition box.

FIG. 21 illustrates the benefits of the present invention in resolvingdifferent multipath environment problems. The multipath environments,discussed in the background previously, including ground reflection,dominant single reflector, etc., are shown in the vertical left column.The user type and code type are shown across the top of the table,including users that are stationary versus mobile, and using C/A-code,M-code, etc. As described previously, the reference “Q1” refers toapplication of the first principle of the QMFR technique, namely therecognition of the quadrature condition having occurred for optimalcurve fitting. Similarly, “Q2” refers to the second QMFR principle,namely that the quadrature condition can be forced via multi-frequency(MF) transmission. The table indicates in which instances and for whichmultipath problems, Q1 and Q2 are found to be faster than averaging,better at lower elevation angles, near instantaneous high accuracymeasurements, or no benefit. As shown in the table, each of the variousbenefits, together with the Q1 and Q2 principles are illustrated.

In FIG. 22, recommended approaches for multipath mitigation, in additionto narrow correlation are shown. Again, the multipath problems arelisted down the left vertical column and the user type and code typesare listed across the horizontal top column. In FIG. 22, some of thecombinations are recommended for QMFR, others are recommended for acombination of multipath-resistant antenna technology (ANT) togetherwith QMFR, and still others are recommended with inertial aiding (IA)with or without QMFR, as shown.

From a review of the detailed description herein, the artisan canrecognize that using QMFR provides substantial advantages in multipathmitigation for GPS users. The first principle of QMFR, noted herein asQ1, recognizes the quadrature condition between direct and delayed pathand offers a faster means to high-accuracy than smoothing multi-patheffects. The second QMFR principle, Q2, forces the quadrature conditionto occur via multiple frequency transmission, and provides nearinstantaneous high-accuracy measurements under many multipathconditions. QMFR, which combines Q1 and Q2, gives users significantlyimproved accuracy in many multipath environments, as illustrated inFIGS. 21 and 22. Further, QMFR, when transmitted via a UHF-bandpseudolite, supports high-accuracy navigation in urban canyons andinside of buildings.

Results applicable to urban environments and inside buildings can beseen in FIG. 23, where QMFR experimentation results are shown withrespect to ITT's Small Unit Operation Situation Awareness System (SUOSAS or SUO) provided under contract to the Defense Advanced ResearchProjects Agency (DARPA). The experimental results show the QMFR providesaccurate ranging with multipath as near in as ¼ chip (chip rate of 32Mcps). In FIG. 23, the delay calibration accuracy was plus or minus 0.5ns. Some differences between SUO and GPS are small unit operation signallevels being much higher due to closer range (1 km v. 10,000 km) and SUOsignals being at UHF-band rather than L-band. This means that directpath signals can pass completely through some types of buildings andusers can receive SUO signals even inside of buildings. Further, whenusing pseudolites, strong local area GPS signals can be provided withmultiple frequencies to force the quadrature condition. Also, theUHF-band can be used for pseudolite signals to obtain better performancein urban environments. Both the urban and building environments areillustrated in FIGS. 21 and 22, as previously described.

The present invention can find application in a wide variety ofdifferent applications. First, receivers which detect the quadraturecondition between the direct path of a delayed path (i.e., the multiplepath) for the purpose of achieving better TOA measurement accuracy whenthe quadrature condition occurs, have immediate impact using the presentinvention. In fact, the normal motion of the satellites or motion of thereceiver that causes variation in the phase angle between the direct anddelayed paths can now be exploited to locate a quadrature or nearquadrature condition and then perform TOA estimation with high-accuracy.

Further, transmitters which force the quadrature condition to occur alsohave application in the preferred embodiment of the present invention.By forcing the quadrature condition to occur, the following transmittersallow suitably equipped receivers to achieve high TOA measurementaccuracy nearly instantaneously with multipath delay as close as ¼ chip(chip duration varying according to the code):

(i) satellite transmitter of C/A-code with frequency hopping over F_(c)plus or minus 1 MHz;

(ii) satellite transmitter of P(Y)-code with frequency hopping overF_(c) plus or minus 5 MHz;

(iii) satellite transmitter of M-code with frequency hopping over F_(c)plus or minus 2.5 MHz;

(iv) pseudolite transmitter in the L-b and of any of the GPS codes withfrequency hopping, but especially of M-code with frequency hopping overF_(c) plus or minus 2.5 MHz;

(v) pseudolite transmitter in the UHF-band of any of the GPS codes withfrequency hopping, but especially of M-code with frequency hopping overF_(c) plus or minus 2.5 MHz. The UHF band is chosen because of itssuperior penetration through and into buildings. This facilitatesreception of GPS signals in urban canyons and inside buildings.

Finally, the present invention may find application in receivers whichfrequency hop along with the preceding transmitters, deciding whichfrequency hop comes closest to the quadrature condition, and thencarrying out leading edge curve fitting to achieve high accuracy TOAestimation.

The receivers and transmitters described above are applicable forimproved performance against multipath in all environments where GPS isused, including terrestrial, marine, airborne and space.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A Direct Sequence Spread Spectrum (DSSS) transmitter comprising: asignal input to receive a first signal; an encoder to accept the firstsignal and to encode the first signal by spread spectrum encoding; atransmitter to transmit the spread-spectrum encoded first signal at aseries of frequencies such that the spread of these frequencies issufficient to force at least a near-quadrature condition to occur in theevent of a multipath condition; the transmitter transmitting over atransmission channel whereby the transmitted encoded first signal issubject to the multipath condition and yielding a direct path signal anda multipath signal; whereby said near-quadrature condition is acondition of a phase of the multi-path signal relative to the directpath signal.
 2. A Direct Sequence Spread Spectrum (DSSS) transmitteraccording to claim 1, wherein: the spread of said series of frequenciesis at least Δf=¼Δt, where Δf is the spread and Δt is a delay associatedwith the multipath condition.
 3. A Direct Sequence Spread Spectrum(DSSS) transmitter comprising: an encoder to spread an informationsignal by encoding the information signal over a spread spectrum; atransmitter to transmit the already-spread first signal at a number ofdifferent carrier frequencies such that a spread of these carrierfrequencies provides a near-quadrature condition to occur between thephases of direct path and multipath signals transmitted from saidtransmitter.